In this 2008 file photo, Gen. Lloyd Austin talks during an interview with reporters in Baghdad, Iraq. The top American general in Iraq said Monday the country will face some 'turbulence' after American forces.

Baghdad, Iraq — As American forces finalize their departure from Iraq, a top challenge for Iraqi security forces will be curbing the continued presence of Sunni and Shiite militant groups, said the top US general in Iraq.

Gen. Lloyd Austin told journalists today that troops "worked as hard as we could for as long as we could" to train Iraqi military and security forces for a "historic transition" that will see all remaining US units gone by the end of the year.

Already, fewer than 20,000 American troops are in Iraq, down from a high of more than 170,000. They are packing their bags and hardware on just eight bases that remain, from the peak of 505 across Iraq. US troops have been virtually absent from Iraqi towns since 2009, so most Iraqis will notice little difference when their government assumes full control.

After nearly nine years of US efforts to stabilize Iraq, the outgoing American commander warned the Iraqi government about continued dangers from Al Qaeda in Iraq, as well as Iran-backed Shiite insurgent groups – the same players that have perennially dogged US forces in Iraq.

"As we leave, you can expect to see some turbulence in security initially, and that's because you'll see various elements try to increase their freedom of movement and freedom of action," said General Austin. "Al Qaeda will be one of those elements.

Violence down from peaks

Despite a string of car bombs and attacks in recent months – 33 Iraqis were killed and 99 injured in the past week, according to a security assessment by the firm AKE – overall levels of violence are far below those of years past.

Austin said that Iraq now "has the opportunity to become a leader in the region, if it chooses to stay on the right path."

Yet Iraq still must overcome big challenges that range from militia attacks and a flow of weapons from Iran into Iraq, to disputed areas like Kirkuk that are "generational in nature."

Austin listed three "Iran-backed" Shiite militias that primarily target US troops, as well as Sunni extremists of Al Qaeda in Iraq (AQI),which once routinely used spectacular suicide bombings to target civilians, the Shiite-led government, and Shiite shrines.

"We expect that Al Qaeda will continue to do what it's done in the past, we expect it's possible they could even increase in inner capability," depending on "how effectively the Iraqi security forces and the government of Iraq are able to focus on that network," said Austin.

He also warned that the Iraqi government – with defense, interior, and intelligence ministries heavily dominated by Shiites – also had to take on Shiite militants of Kataib Hezbollah, Asaib Ahl al-Haq, and the Promised Day Brigades.

Those Iran-supported groups increased their attacks in the first half of 2011, according to official tabulations and reports, and oversaw a "sharp increase" in munitions crossing the border from Iran.

Austin said the "stream of lethal accelerants from Iran" such as rockets and specialty roadside bombs was continuing, but that US and Iraqi interdiction efforts had "some success."

Hezbollah-like groups emerging?

Shiite groups are "focused on creating a Lebanese Hezbollah kind of organization in this country, a government-within-a-government, and those elements would have their own militia," Austin stated. "As we leave, if these elements are left unchecked, they will then eventually turn on the [Iraqi] government. And they should be concerned about that."

Iraqi security forces were "approaching" the ability to control the "internal security environment," Austin said.

"No doubt there will be a gap, but that is normal. We will fill the gap," the first deputy of the Ministry of Interior, Adnan al-Asadi, said in Baghdad.

"The Iraqi forces have gained strength and experience and have held the ground, for how long now?" said Mr. Asadi. "How many American soldiers do you see on the streets these days? How long has it been since you last saw American soldiers on the streets? That answers your question."

Austin said that Iraq was working to develop a "healthy relationship" with neighboring Iran, and noted that much of the Iraqi leadership "don't see an enormous threat from Iran at this point."

Austin said that Iraq had to "continue to apply pressure on the violent extremist networks," as well as improve training for its security forces, improve the rule of law, and focus on providing essential services.

The US has spent more than $50 billion on infrastructure and other projects in Iraq – many of them unknown to average Iraqis as American-funded efforts, their US money masked to avoid security problems. But Iraqis are still plagued by lack of regular electricity, clean water, and other key services.

Iraq "will have to strike a balance," said Austin. "At the end of the day, I think the Iraqis will do what is best for Iraq. They'll make their own decisions."